Neural signatures of indirect pathway activity during subthalamic stimulation in Parkinson’s disease

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) produces an electrophysiological signature called evoked resonant neural activity (ERNA); a high-frequency oscillation that has been linked to treatment efficacy. However, the single-neuron and synaptic bases of ERNA are unsubstantiated. This study proposes that ERNA is a subcortical neuronal circuit signature of DBS-mediated engagement of the basal ganglia indirect pathway network. In people with Parkinson’s disease, we: (i) showed that each peak of the ERNA waveform is associated with temporally-locked neuronal inhibition in the STN; (ii) characterized the temporal dynamics of ERNA; (iii) identified a putative mesocircuit architecture, embedded with empirically-derived synaptic dynamics, that is necessary for the emergence of ERNA in silico; (iv) localized ERNA to the dorsal STN in electrophysiological and normative anatomical space; (v) used patient-wise hotspot locations to assess spatial relevance of ERNA with respect to DBS outcome; and (vi) characterized the local fiber activation profile associated with the derived group-level ERNA hotspot.

Intro: "Parkinson's disease (PD) is a common movement disorder that occurs after degenerafion of nigrostriatal dopaminergic projecfions.." this isn't quite accurate… the motor signs occur after nigrostriatal degenerafion, but the disorder starts much earlier than that.
"An integrafive account of these synapfic circuit acfivafions and their importance for neuronal circuit engagement in the subcortex is missing; which mofivated the contextualizafion of these mechanisfic phenomena within this work" This should be said in a simpler way

Results/figures
The authors emphasize that sfimulafion results in "pafterned" inhibifion of STN neuronal firing.The specific paftern generated should be stated.Is that suppression of STN firing at a parficular fime point after a sfimulafion pulse, but not at other fime points, is crifical for the emergence of ERNA?If so it needs to be explained more clearly as this pafterning received a lot of emphasis throughout the paper.
Figure 3D -this schemafic seems like it would be very useful for understanding the mental model of how ERNA comes about from synapfic interacfions.I tried to understand it but failed.The three parts I, ii, and iii are probably sequenfial fime points but this isn't specified.In the legend, it wasn't clear to me what comments referred to 3D versus 3E. the exact meaning of the lefters a and b in the figure aren't clear to me.The schemafic needs to be simplified and/or explained much befter.5c -the orientafion of the STN is confusing.The "dorsolateral" arrow suggests that this is a coronal view (since the coronal plane contains a dorso-ventral axis and medial-lateral axis).But the STN is not oriented the way one would expect in the coronal plane (would be obliquely).
5f-not menfioned in the legend.The results do describe it: "Fibers crossing through the group level ERNA hotspot (1 mm radius) consisted of 30% GPe-STN, 30% STN-GPe, 12% SMA-STN, 10% premotor-STN, and 19% M1-STN fibers".From results and legend to fig 5, its not clear where these percentages come from.The methods refers to a 'fiber atlas" but very liftle informafion is given to understand how to interpret 5f and at minimum the derivafion of these from a specific fiber atlas should be menfioned when giving those results and some statement of what that atlas means (was it from human histology, for example) Discussion: The authors emphasize the point that ERNA is linked to the therapeufic outcome of DBS, and indicate that modulafion of the indirect pathway is important in the therapeufic mechanism, but don't really explain the exact link between ERNA and therapeufic benefit.Does ERNA itself produce a downstream effect that is responsible for the efficacy of STN DBS?Or is it just a marker for engagement of a populafion of GPe cells that themselves are important for the therapeufic mechanism?Further, could ERNA produce the beta suppression that is a known hallmark of therapeufic DBS?The answers of course require speculafion but is worth doing here given the very detailed dissecfion of basal ganglia microcircuitry.
A recent paper showed that ERNA occurs in dystonia, which seems to suggest it is related to the generic basal ganglia architecture rather than to specific parkinsonian derangements (such as increased beta band acfivity).Could the presence of ERNA in dystonia indicate that therapeufic DBS in that condifion must also engage similar synapfic mechanisms in GPe?This is worth a comment in the discussion.
There are many sentences that are too long, and are hard to follow, some examples below: "The fact that ERNA can be monitored using the macrocontacts of DBS electrodes during clinicallyrelevant sfimulafion highlights the translafional relevance of the underlying synapfic / neuronal circuit signature as suggested by the findings presented in this study" -not sure what this sentence means "Our finding that pafterned neuronal inhibifion is temporally locked to peaks of the ERNA waveform provides a high spafiotemporal resolufion neurophysiological readout that not only implicates the subcorfical orchestrafion of ERNA, but also substanfiates the hypothesis that this electrophysiological signature is a representafion of GPe-mediated inhibifion" -this sentence is very hard to follow "While convergence of indirect vs hyperdirect pathway fibers at the level of STN has been shown to be instrumental in the orchestrafion of pathological subcorfical oscillatory acfivity and neuronal synchrony, there has been debate about the level of discrefizafion of respecfive fiber acfivafions during subthalamic DBS" -by "level of discrefizafion" do you mean whether all pathways are acfive or just some?The sentence is unclear.
Reviewer #2 (Remarks to the Author): Steiner and colleagues explored micro and macro scale origins of ERNA-an important physiological marker of deep brain sfimulafion efficacy when applied to the STN.Cellular origins of this physiological marker sfill remains unknown and could potenfially inform DBS targefing and lead to development of new sfimulafion approaches.This is a well wriften study which presents a novel hypothesis and would be suitable for publicafion after major revisions.
My only feedback to the authors is regarding their computafional model which is an important element of this study tying micro electrode recordings to mapping in normafive anatomical space.
(1) Authors used an integrate and fire model to represent 100 STN and 100 GPE neurons.In this parficular context, when their main hypothesis is the engagement of the indirect pathway (which would hyperpolarize STN neurons), I would have expected the authors to ufilise a STN model that can elicit rebound excitafion after hyperpolarizafion (Bevan et al 2000, Terman et al 2002).The authors should either update their theorefical approach or convincingly mofivate their modelling choices (i.e.why in this experimental context, we should not be expecfing rebound acfivafion).
(2) In the absence of the computafional model exhibifing appropriate membrane properfies, it becomes difficult to be convinced by the following statement "In parficular, the inifial peak is conceptualized to be the result of sustained GPe-mediated inhibifion to STN, whereas the second peak is the result of synapfic compefifion at the level of GPE which feeds back to STN to produce recurrent inhibifion."which enfirely relies on the computafional model.

NCOMMS-23-25757 [R1]
(Neural signatures of indirect pathway activity during subthalamic stimulation in Parkinson's disease) The authors would like to thank the Editorial team and Reviewers for providing us the opportunity to improve the manuscript content and data presentation through insightful feedback.Point by point responses are found below.

Responses to Reviewer #1
Overview: This is a very interesting and very original paper that seeks to elucidate the synaptic and axonal mechanisms of an important and recently discovered phenomenon, Evoked Resonant Neural Activity (ERNA).The authors are uniquely positioned to do this study, leveraging their intraoperative recording set up that is customized to provide independent control of two closely spaced microelectrodes.This allows recording of both LFP like phenomena, and single units, in close proximity to another electrode that provides the stimulus.The computational and imaging components add to the paper but the physiology is the most novel element.There are a lot of great data here and the fact that the authors took a "deep dive" on understanding the basis for ERNA is much appreciated.There are several general issues that reduce the value of the paper in its current form, and a number of specific, more minor ones.
Thank you for your kind appraisal and thoughtful comments, which helped us to improve the quality of our manuscript and data presentation. Major: 1) At many points in the paper, especially discussion, the language is complex, with long sentences whose exact meaning is elusive (example below).The authors need to rewrite, these parts to more clearly elucidate their key points in a more accessible way with cleaner, simpler language.
Thank you for this important feedback.

Amendments to the manuscript:
We have rewritten various sentences outlined by the reviewer (see below; in responses to "comments on specific sections").We have moreover edited the entire manuscript to convey key messages in clearer and simpler language.These general edits throughout can also be identified by blue text within the manuscript.
2) A major point of the paper is that ERNA is a property of the indirect intrinsic basal ganglia pathway.But, in both 3D and 4A, the stimulation of hyperdirect inputs to STN places a role in the model and thus would seem to play a role in generating ERNA.This seems to contradict the point that that ERNA is a property of the indirect pathway (which classically is striatal D2 to GPe to STN to GPi).Could ERNA still happen without hyperdirect inputs?If so why does the hyperdirect input appear to play a role in the author's model of ERNA?
The reviewer raises an important point regarding the relevance of hyperdirect pathway inputs for the generation of ERNA.Hyperdirect pathway inputs to STN have been incorporated into our model to provide an anatomically accurate representation of the circuit architecture.Because activation of cortical afferents to the STN is a phenomenon that occurs during STN-DBS, these fibers were not omitted from the model.However, an important detail which is supported by empirical data in STN slices (Steiner et al., J Neurosci, 2019) and is incorporated into the model is that cortical inputs to the STN rapidly and robustly depress during high-frequency stimulation (outlined in Fig. 4B; please note that other synaptic currents are able to maintain neurotransmitter release during high frequency activations, as per implicated references).Thus, despite the physical presence of cortical axonal inputs, the functional contribution to the produced synaptic current in STN is effectively negligible after the first few impulses due to inherent functional properties of these synapses (rapid depression).
Inspired by the Reviewer's comment, we have used our model to probe the relevance of hyperdirect inputs in more detail (and conceptualize the aforementioned points).To this end, we have implemented a new in silico condition in which hyperdirect inputs are omitted.In this condition, ERNA peaks show similar dynamics as compared to rapidly depressing hyperdirect inputs.This new implementation demonstrates that hyperdirect pathway depression during high-frequency stimulation has a similar effect as outright hyperdirect pathway omission; therefore, reinforcing the point that hyperdirect pathway inputs are of negligible importance towards the generation of the ERNA waveform.

Amendments to the manuscript:
The following text has been added to the Discussion (Pg.14/15): "Because of the rapid depression of hyperdirect inputs during HFS, the effect of hyperdirect pathway stimulation in the generation of ERNA can be considered negligible.This is corroborated by the in silico findings presented in this study, which show very similar dynamics of ERNA in the absence of hyperdirect inputs as compared to rapidly depressing hyperdirect inputs (Supplementary Fig. 3)".
We have also added this implementation to the Supplementary Material: Supplementary Figure 3 -Modeling results in the presence and absence of rapidly depressing hyperdirect inputs to STN. (Upper panels) STN conductances in response to HFS. (Lower panels) In silico dynamics of first (dark blue) and second peak (light blue) of the ERNA waveform.Note that ERNA peaks show similar dynamics in the (i) presence and (ii) absence of rapidly depressing hyperdirect inputs to STN.This suggests that hyperdirect pathway inputs are likely of negligible importance towards the generation of the ERNA waveform.
3) The authors make the important point that ERNA can also be observed during pallidal DBS, not just subthalamic.The introduction and discussion indicate that involvement of GPe is likely a common mechanism of both.Could they speculate more on the synaptic mechanism of pallidal stim?[for simplicity, we have broken this comment up into three parts, retaining black text for Reviewer questions and blue text for answers] In a recent scientific commentary (Steiner & Milosevic. Brain Comms, 2023) and review article (Neumann, Steiner, Milosevic. Brain, 2023) we proposed that, similar to STN ERNA, the initial peak of GPi ERNA is a result of direct activation of afferent inputs (producing a net hyperpolarization, due to the greater abundance of GPe inputs compared to STN).Concurrent to the activation of afferent inputs, we proposed that GPi-DBS may additionally (through invasion of axon collaterals and fibers of passage) invade the same circuit that is responsible for the production of STN ERNA (i.e., reciprocal connections between STN and GPe).Through antidromic activation of GPe and STN afferents, and eventual invasion of axon collaterals, GPi DBS can produce neurotransmitter release at remote sites (i.e., STN-mediated release of glutamate in GPe, and GPe-mediated release of GABA in STN).The excitation of GPe would thereafter lead to recurrent inhibition of GPi (and STN), and therefore, the second peak of the ERNA waveform.In STN, each stimulus would produce a net inhibitory response in STN, as well as concurrent excitation of globus pallidus externus (GPe), resulting in feedback inhibition in STN.The same is hypothesized for GPi ERNA via invasion/activation of collateral projections and axons of passage of the reciprocal STN-GPe connectivity.Thus, each of the ERNA waveform peaks is likely a substrate of inhibitory input via GPe.An important note is that the spike firing patterns in STN are only achieved when using subthreshold stimulation amplitudes, which do not cause complete suppression of neuronal firing.Highfrequency stimulation (HFS) at clinically relevant intensities would result in the complete suppression of firing and the elicitation of large amplitude ERNA waveforms.In GPi, patterned firing seems to manifest when spike firing re-emerges (…) after depression of striatal GABAergic inputs, likely unmasking inhibitory-excitatory GPe-STN competition (…).
Additional support for the suggested mesocircuit activation comes from observations that: (1) effective GPi-DBS produces inhibition of neuronal firing in STN (hypothesized to be mediated by GPe; Liu et al., J Neurophys, 2012); and (2) STN-DBS produces ERNA in GPi (Schmidt et al., Brain Stim, 2020).
While the main focus of the present work is on mechanistic aspects of STN ERNA, we have nevertheless added further speculative details about GPi ERNA per the request of the Reviewer.

Amendments to the manuscript:
The following text has been added to the Discussion (Pg.15): "As such, GPi DBS may in fact also invade the reciprocal STN-GPe mesocircuit network which underlies the generation of ERNA.In GPi, the initial ERNA peak is likely a result of direct activation of afferent inputs (producing a net hyperpolarization, due to the greater abundance of GPe inputs compared to STN).Through concurrent antidromic activation of GPe and STN afferents, and invasion of axon collaterals, GPi-DBS can produce STNmediated release of glutamate in GPe, and GPe-mediated release of GABA in STN.The excitation of GPe would thereafter lead to recurrent inhibition of GPi, and therefore, the second peak of the ERNA waveform." Is it that the therapeutic target of pallidal stim is necessarily GPe (eg the therapeutic contact should be in or at the border of GPe) or is retrograde activation of GPe-GPi fibers more likely to engage the relevant GPe synapses?
Thorough scrutinization of synaptic activation responses in GPi (e.g., ERNA & striatalmediated responses, etc.) is presently a topic of ongoing investigation in the lab, in which we plan to relate evoked responses to stimulation location (to be able to answer the Reviewer's question directly in subsequent work).At present however, we are happy to speculate.As is outlined in the above amendment, we suspect that it is the result of retrograde activation of fibers from GPe-to-GPi (which have collaterals to STN) and STNto-GPi (which have collaterals to GPe).Optogenetics works have indeed suggested that global activation of GPe does not produce therapeutic benefit in parkinsonian rodents (Mastro et al., Nat Neurosci, 2017), whereas cell-type specific GPe interventions do, which (speculatively) may be achieved by DBS through retrograde activation of GPe-GPi fibers.

Amendments to the manuscript:
As above (Pg.15), in addition to the following clarification (Pg.15): "Moreover, subsequent optogenetic works have also shown that direct somatic inhibition of STN also produced antiparkinsonian benefits, 45 and that activation of populations of GPe neurons that selectively project to STN could also produce long-lasting antiparkinsonian benefits (whereas global activation of GPe did not). 46" Can they account for fact that pallidal ERNA is lower amplitude than subthalamic ERNA?This is indeed a very interesting question.At present, we can only speculate about these relative differences.We hypothesize that differences in ERNA amplitudes may be a reflection of the relative innervation strengths from GPe. STN receives the vast majority of its GABAergic input from GPe (Bevan et al., Prog Brain Res, 2007) while the GABAergic input to GPi is more mixed, containing inhibitory inputs from striatum, GPe, and mutual synaptic connectivity.Greater relative innervation of GPe-to-STN as compared to GPe-to-GPi may therefore be the reason for greater ERNA amplitudes in STN.

Amendments to the manuscript:
The following text has been added to the Discussion (Pg.15): "A greater relative innervation of inhibitory inputs from GPe-to-STN as compared to GPe-to-GPi (which also receives inhibitory inputs from other sources; namely, striatum) may explain observations of greater ERNA amplitudes in STN compared to GPi. " Comments on specific sections: Intro: "Parkinson's disease (PD) is a common movement disorder that occurs after degeneration of nigrostriatal dopaminergic projections.." this isn't quite accurate… the motor signs occur after nigrostriatal degeneration, but the disorder starts much earlier than that.
Thank you for the attentive comment.

Amendment to the manuscript:
The following clarification has been made (Pg.3): "Parkinson's disease (PD) is a common movement disorder that occurs after is associated with progressive degeneration of nigrostriatal dopaminergic projections." "An integrative account of these synaptic circuit activations and their importance for neuronal circuit engagement in the subcortex is missing; which motivated the contextualization of these mechanistic phenomena within this work" This should be said in a simpler way Thank you for the attentive comment.

Amendment to the manuscript:
The following clarification has been made (Pg.3): "An integrative account of these synaptic subcortical circuit activations and their importance for neuronal circuit engagement in the subcortex is missing, which motivated the contextualization of these mechanistic phenomena within this work presented is this study."

Results/figures:
The authors emphasize that stimulation results in "patterned" inhibition of STN neuronal firing.The specific pattern generated should be stated.
Thank you for the attentive comment.We now explicitly specify the pattern of inhibition of STN neuronal firing.

Amendment to the manuscript:
The following clarification has been made to the Results (Pg.10): "These data demonstrate a relationship between the shape of the ERNA waveform and the similarly patterned neuronal suppression in the interstimulus interval (inhibition of neuronal activity that is time locked to peaks of the ERNA waveform, whereby the strength of the inhibition is proportional to size of the respective peak)." Is that suppression of STN firing at a particular time point after a stimulation pulse, but not at other time points, is critical for the emergence of ERNA?If so it needs to be explained more clearly as this patterning received a lot of emphasis throughout the paper.
The data presented in this paper suggests that the ERNA waveform is a readout of the recurrent synaptic input activation of indirect pathway fibers.STN neuronal suppression during HFS is a consequence of GPe-mediated inhibitory activations (i.e., ERNA) and as such, each peak of the ERNA waveform produces inhibition of neuronal firing (Fig 2 of manuscript).STN firing itself, while being pattered by ERNA (i.e., subject to recurrent inhibition), is not essential for the emergence of ERNA.In fact, when ERNA (inhibitory signature) amplitude is large, neuronal firing is more supressed; thus, large amplitude ERNA waveforms impose potent inhibition, and therefore exist in the complete absence of spiking.This is supported by data displayed in Figure 2 that suggests that stronger neuronal suppression is correlated with higher amplitudes of ERNA.Findings of greater ERNA amplitudes being related to better therapeutic results (e.g., Sinclair et al., Ann Neurol, 2019) likely suggest stronger entrainment of GPe-mediated inhibition.

Amendment to the manuscript:
The following clarification has been made in the Discussion (Pg.13): "Our finding that the ERNA waveform peaks are temporally locked to inhibitions of STN single-neuron activity that patterned neuronal inhibition is temporally locked to peaks of the ERNA waveform provides a high spatiotemporal resolution neurophysiological readout that not only implicates the subcortical orchestration of ERNA, but also substantiates the hypothesis that ERNA this electrophysiological signature is likely a GPe-mediated signature (given that GPe is the major source of inhibition to STN)." Figure 3D this schematic seems like it would be very useful for understanding the mental model of how ERNA comes about from synaptic interactions.I tried to understand it but failed.The three parts i, ii, and iii are probably sequential time points but this isn't specified.In the legend, it wasn't clear to me what comments referred to 3D versus 3E. the exact meaning of the letters a and b in the figure aren't clear to me.The schematic needs to be simplified and/or explained much better.
Thank you for the opportunity to clarify.1a and 1b happen simultaneously, hence use of the same number.1a: Recruitment of inputs to STN (cortical and GPe; the net effect is sustained inhibition in STN, i.e., ERNA P1); 1b: Orthodromic recruitment of excitatory STN-GPe efferents & antidromic recruitment of inhibitory GPe-STN afferents with invasion of GPe-GPe axon collaterals (producing synaptic competition at GPe, giving rise to net excitation of increasing strength).2: This recurrent inhibition in STN is the result of an emerging net excitation in GPe (i.e., 1b), which feeds forward to STN (producing ERNA P2).

Amendments to the figure:
 amended all subheadings for clarity (provided explicit explanations wherever possible)  specified that 1a and 1b occur simultaneously  colour-coded the two distinct timepoints represented within each interstimulus interval  amended figure legend text … (D) Conceptual schematic to illustrate how STN stimulation may trigger a cascade of synaptic events that may ultimately give rise to the resonant ERNA peak.Left: Schematic representation of activated fibers at two timepoints during a single interstimulus interval (1a/b & 2).Note that (1a) and ( 1b) are expected to occur simultaneously within a "monosynaptic" time course in response to individual stimuli, whereas (2; blue shading) occurs subsequently/consequently, at a "disynaptic" time course.Right: These synaptic responses change in amplitude across successive interstimulus intervals as a result of short-term synaptic plasticity.Vertical lines represent individual stimuli at HFS. Blue positive-going potentials in STN correspond to direct inhibitory afferent activations (1a blue), whereas negative-going red potentials represent activations of excitatory cortical inputs (1a red).1a: sustained GPe transmission paired with rapidly depressing cortical transmission leads to a sustained net inhibition in STN (i.e., P1 of STN ERNA, filled grey).1b: sustained GPe transmission paired with even more sustained STN transmission leads to an increasing net excitation in GPe.2: the increasing net excitation in GPe produces recurrent inhibition of STN (i.e., P2 of STN ERNA, filled black).
5c -the orientation of the STN is confusing.The "dorsolateral" arrow suggests that this is a coronal view (since the coronal plane contains a dorso-ventral axis and medial-lateral axis).But the STN is not oriented the way one would expect in the coronal plane (would be obliquely).
Thank you for this feedback.We agree that the previously used 2D arrow was insufficient to facilitate understanding of the orientation.

Amendment:
We have incorporated 3D orientation arrows.
(Figure 5C) Group data (n = 19; postoperative neuroimaging was unavailable for one patient) of the ERNA amplitude heatmap in MNI space.orientation arrows -d: dorsal; m: medial; p: posterior 5f-not mentioned in the legend.The results do describe it: "Fibers crossing through the group level ERNA hotspot (1 mm radius) consisted of 30% GPe-STN, 30% STN-GPe, 12% SMA-STN, 10% premotor-STN, and 19% M1-STN fibers".From results and legend to fig 5, its not clear where these percentages come from.The methods refers to a 'fiber atlas" but very little information is given to understand how to interpret 5f and at minimum the derivation of these from a specific fiber atlas should be mentioned when giving those results and some statement of what that atlas means (was it from human histology, for example) Thank you for the attentive comment.We implemented the Petersen atlas that draws from human histological and structural MRI data and has been further refined by expert neuroanatomists (Petersen et al, Neuron, 2019).In 5f, the relative percentages of sensorimotor hyperdirect and indirect pathway fibers passing through the ERNA hotspot were reported as a percentage of the total number of fibers passing through the ERNA hotspot.

Amendment to the manuscript:
The following clarification has been made in the Methods (Pg.9): "In a final step, the fiber activation profile 38 of the group level ERNA hotspot (1 mm radius) was characterized using the subcortical Petersen fiber atlas (human histological and structural MRI data which has been further refined by expert neuroanatomists), 25 which considered M1 (face, upper extremity, and lower extremity), SMA, and premotor hyperdirect pathways, as well as GPe-STN and GPe-STN indirect pathway fibers (Fig. 5E).Relative percentages of the sensorimotor hyperdirect and indirect pathway fibers passing through the ERNA hotspot were reported (Fig. 5F)."Discussion: The authors emphasize the point that ERNA is linked to the therapeutic outcome of DBS, and indicate that modulation of the indirect pathway is important in the therapeutic mechanism, but don't really explain the exact link between ERNA and therapeutic benefit.Does ERNA itself produce a downstream effect that is responsible for the efficacy of STN DBS? [for simplicity, we have broken this comment up into three parts, retaining white text for Reviewer questions and blue text for answers] Thank you for raising these relevant questions.Our work suggests that at the level of STN, ERNA likely represents recurrent synaptic inhibition via GPe, as shown in Figure 2.While the amount of inhibition of neuronal firing has been previously shown to be associated with the therapeutic threshold of STN-DBS (Milosevic et al. JNNP, 2019), the more widespread cortico-basal-ganglia network effects are yet to be evaluated in detail.Recent work from the group of Andreas Horn has suggested that an overlap exists in the cortical functional (fMRI-based) connectomic profiles associated with STN-and GPi-DBS.We hypothesize (Steiner & Milosevic, Brain Comms, 2023) that this high-latency functional overlap in cortex is a downstream / feedforward phenomenon driven by GPe-mediated inhibition throughout the cortico-basal ganglia network, since (i) ERNA is common signature to both STN-and GPi-DBS (Johnson et al., Brain Comms, 2023), whereas (ii) fast-latency (antidromic-driven) cortical activations do not occur during effective GPi-DBS (Johnson et al., J Neurosci, 2020).

Amendment to the manuscript:
The following clarification has been made in the Discussion (Pg.15): "It is perhaps this resonant subcortical mesocircuit phenomenon, which promotes GPemediated resonant downstream inhibition throughout the broader basal ganglia network, that also underlies the slower timecourse anticorrelation observed within the convergent functional connectomic profile associated with both of these interventions. 47" Or is it just a marker for engagement of a population of GPe cells that themselves are important for the therapeutic mechanism?Importantly only cell type specific, but not global stimulation of GPe produces therapeutic benefit, which has recently confirmed by preclinical studies.In the 6-OHDA model of PD, optogenetic GPe modulation has been shown to produce therapeutic benefit in a cell-type specific manner (when increasing activity of PV+, but not Lhx6 neurons; Mastro et al, Nat Neurosci, 2017).Interestingly, PV+-neurons have been shown to preferentially target STN (Mastro et al, J Neurosci, 2015).Thus, stimulation in areas of STN that receive projections from these cells may produce similar circuit activation responses.These areas may be able to be identified by eliciting large amplitudes of ERNA.

Amendment to the manuscript:
The following clarification has been made in the Discussion (Pg.16): "Moreover, targeting of the subcortical network that gives rise to ERNA may enable further optimization of STN DBS for therapeutic purposes.Given that It has been shown that the activation of GPe neuronal populations that preferentially project to STN can lead to longlasting therapeutic benefits that persist beyond stimulation cessation,. 46our findings may provide a means of selectively targeting these therapeutically-relevant GPe fibers." Further, could ERNA produce the beta suppression that is a known hallmark of therapeutic DBS?The answers of course require speculation but is worth doing here given the very detailed dissection of basal ganglia microcircuitry.Indeed, we hypothesize that GPe-mediated inhibition (which is our proposed electrophysiological basis for ERNA) likely also contributes to the suppression of beta oscillations.Beta oscillations are thought to critically depend on pallidal inputs to STN that are in antiphase to cortical inputs (Baufreton et al., J Neurosci, 2005;Cagnan et al., Brain, 2015); whereas our understanding of DBS is that it results in the depression of cortical inputs and entrainment of GPe inputs to STN.Thus, these pathway modulations likely cumulatively contribute to the disruption and suppression of circuit phenomena necessary to produce pathological beta oscillations (Steiner et al., J Neurosci, 2019).

Amendment to the manuscript:
The following clarification has been made in the Discussion (Pg.16): "These observations collectively suggest that subthalamic DBS likely engages the same subcortical mesocircuit network that gives rise to beta oscillations, imposing stimulationinduced suppression of pathological activity through high frequency driving of GPemediated inhibition (i.e., ERNA) in tandem with suppression of cortical influence on STN but has the capability to exploit the very same circuit motif to produce a therapeutic neurocircuit intervention that is reflected in ERNA." A recent paper showed that ERNA occurs in dystonia, which seems to suggest it is related to the generic basal ganglia architecture rather than to specific parkinsonian derangements (such as increased beta band activity).Could the presence of ERNA in dystonia indicate that therapeutic DBS in that condition must also engage similar synaptic mechanisms in GPe?This is worth a comment in the discussion.Indeed, Wiest et al showed that ERNA can be elicited by STN-DBS in dystonia (Wiest et al., Mov Disord, 2023).We agree with the reviewer that this may be evidence that ERNA is reflective of a synapse-specific circuit-phenomenon rather than being disease specific.

Amendment to the manuscript:
The following text has been added to the Discussion (Pg.15): "Interestingly, ERNA can also be elicited by STN-DBS in dystonia, substantiating that ERNA may be reflective of a synapse-specific circuit-phenomenon rather than being disease specific 56 ." There are many sentences that are too long, and are hard to follow, some examples below: "The fact that ERNA can be monitored using the macrocontacts of DBS electrodes during clinically-relevant stimulation highlights the translational relevance of the underlying synaptic / neuronal circuit signature as suggested by the findings presented in this study"not sure what this sentence means The sentence was initially intended to highlight usability of ERNA as a biomarker of circuit activation.We agree with the reviewer that this sentence is not necessary to motivate the present study.
Amendment to the manuscript: In order to provide a more coherent discussion, we have deleted the respective paragraph from the Discussion (Pg.13).
"Our finding that patterned neuronal inhibition is temporally locked to peaks of the ERNA waveform provides a high spatiotemporal resolution neurophysiological readout that not only implicates the subcortical orchestration of ERNA, but also substantiates the hypothesis that this electrophysiological signature is a representation of GPe-mediated inhibition"this sentence is very hard to follow We agree with the reviewer that this sentence is unnecessarily complicated.

Amendment to the manuscript:
The following clarification has been made (Pg.13): "Our finding that the ERNA waveform peaks are temporally-locked / give rise to inhibitions of single-neuron activity a high spatiotemporal resolution neurophysiological readout that not only implicates the subcortical orchestration of ERNA, but also substantiates the hypothesis that ERNA this electrophysiological signature is likely a GPe-mediated signature (given that GPe is the major source of inhibition to STN)." "While convergence of indirect vs hyperdirect pathway fibers at the level of STN has been shown to be instrumental in the orchestration of pathological subcortical oscillatory activity and neuronal synchrony, there has been debate about the level of discretization of respective fiber activations during subthalamic DBS" -by "level of discretization" do you mean whether all pathways are active or just some?The sentence is unclear.
Thank you for the opportunity to clarify.With the "level of discretization" we wanted to specify that the relative contributions of engaged pathways, as it relates to therapeutic efficacy, is of ongoing debate (Butenko et al. Neuroimage Clin. 2022).However, it is not only important which fibers are activated, but also the functional consequences of such activations (which we believe is the most valuable contribution of our work).

Amendment to the manuscript:
The following clarification has been added to the Discussion (Pg.14): "While convergence of indirect vs hyperdirect pathway fibers at the level of STN has been shown to be instrumental in the orchestration of pathological subcortical oscillatory activity 1,4 and neuronal synchrony, 43 there has been debate about the level of discretization of respective fiber activations during relative contributions of engaged pathways to the therapeutic effect of subthalamic DBS. 6,44However, beyond these structural fiber profiles, we suggest that projection-specific synaptic dynamics have to be taken into consideration to appreciate the functional consequences of such fiber activations (e.g., cortical suppression paired with GPe entrainment)." For reference, below is a figure (and figure legend) from Neumann, Steiner, Milosevic.Brain, 2023 conceptualizing this common network activation phenomenon: Neumann, Steiner, Milosevic (Brain, 2023) -Figure 3 _ Mesoscale effects of DBS.(ii) Subthalamic nucleus (STN) and (iii) globus pallidus internus (GPi) during 100 Hz stimulation, as well as hypothesized circuit activation profiles that would explain the emergence of ERNA (adapted from Steiner et al. 2023, Brain Communications).